The present invention relates to a modulation technique which provides a high data rate through band limited channels, and in particular to an in-band-on-channel (IBOC) FM broadcast modulation system for digital data, especially digital audio.
In the United States, FM broadcasters can transmit information in sidebands within 100 kHz of their assigned carrier frequency at full power, and from 100 kHz to 200 kHz around the carrier at 30 dB down from full power. The standard stereo audio signal is placed in a bandwidth within 53 kHz of the carrier. The broadcaster is, thus, able to transmit other information in the remainder of the bandwidth, subject to the constraints described above.
It has become desirable for FM broadcasters to simultaneously broadcast stereo audio and digital data. The digital data could, for example, represent a high quality version of the stereo audio being broadcast. This requires a relatively high data rate channel which is restricted to a relatively narrow bandwidth. For example, a digital data stream carrying high quality audio can have a bit rate of 128 kilobits per second (kbps). A signal carrying such a data stream cannot be transmitted in the bandwidth available in an FM broadcast signal without some form of compression to decrease the bandwidth required for the signal.
It is always desirable to provide data at higher data rates through channels which have limited bandwidth. Many modulation techniques have been developed for increasing the data rate through a channel. For example, M-ary phase shift keyed (PSK) and Quadrature Amplitude Modulation (QAM) techniques permit compression by encoding a plurality of data bits in each transmitted symbol. Such systems have constraints associated with them. First, the hardware associated with such systems is expensive. This is because these techniques require a high level of channel linearity in order to operate properly. Consequently, extensive signal processing must be performed for carrier tracking, symbol recovery, interpolation and signal shaping. Second, such techniques are sensitive to multipath effects. These effects need to be compensated for in the receiver. Third, these systems often require bandwidths beyond those available in some applications (for example in-band on-channel broadcast FM subcarrier service) for the desired data rates.
In accordance with principles of the present invention, an FM broadcast transmitter transmits a broadcast signal having a carrier at a broadcast frequency and sidebands, able to be transmitted at full power, within a transmission bandwidth around the carrier. It includes a source of a modulated FM stereo signal having a carrier at the broadcast frequency and having sidebands with a bandwidth less than the transmission bandwidth representing a stereo signal. It also includes a source of a modulated IBOC signal, having carrier pulses spaced relative to each other to represent the IBOC digital data signal encoded as a variable pulse width encoded signal, and a bandwidth within the transmission bandwidth not overlapping the FM stereo signal sidebands. A signal combiner combines the modulated FM stereo signal and the modulated IBOC signal to form the broadcast signal.
In accordance with another aspect of the present invention, an FM broadcast receiver receives a broadcast signal including a first modulated signal representing an FM stereo signal, and a second modulated signal, having carrier pulses spaced relative to each other to represent an in-band-on-channel (IBOC) digital data signal encoded as a variable pulse width encoded signal. It includes a signal separator for generating a first separated signal representing the FM stereo signal and a second separated signal representing the IBOC digital data signal. An FM signal processor generates a stereo audio signal represented by the FM stereo signal. An IBOC signal processor generates a digital data signal represented by the IBOC digital data signal.
The technique according to the principles of the present invention provides an FM transmission system which includes a second channel carrying a relatively high data rate digital signal. This channel is placed in the portion of the FM bandwidth which can be transmitted at full power. The circuitry required to implement such a channel is relatively simple and inexpensive. Further, it does not require high channel linearity and is not subject to multipath problems. The additional circuitry necessary to implement this channel in a receiver is relatively small, and may be coupled to the output of the preexisting IF circuit in the receiver.